This invention relates to power supplies and more specifically, to a method and apparatus for reducing audible noise in a power supply transformer.
Today many electronic devices require one or more sources of stable DC voltage. This has lead to a demand for improved power supplies to perform the basic function of rectifying AC voltage to DC voltage, filtering the DC voltage to reduce the undesirable remaining AC portion of the rectified waveform and regulating to insure the output voltage is insensitive to variations in the input voltage.
One type of power supply regulator is a switch mode regulator. Switch mode regulators utilize switching to regulate the amount of energy transferred from the input through an inductor to the output lead. Switch mode regulators tend to be efficient, compact and lightweight.
One drawback of switch mode regulators is that they can produce audible noise. This occurs when the core of an inductor of the transformer in a switch mode power supply changes size when current sent through the inductor coil rapidly changes. This is known as magnetostriction. Because switch mode regulators typically have a periodic current in standby mode, a resonance can occur whereby magnetostriction causes a mechanical interaction between the core and the windings of the inductor that leads to a vibration. This vibration causes an audible noise to emanate from the power supply. This is undesirable, especially when the power supply is in standby mode and is powering in home appliances, such as a television.
Thus, what is needed is a way to reduce audible noise in the transformer of a switch mode power supply.